I. Field of the Invention
The present invention concerns novel lanthionine ketimine compounds, the process of preparing such compounds, and their use. The invention also concerns methods of using lanthionine, pyruvate, lanthionine ketimine, lanthionine ketimine derivatives and combinations of these compounds in the treatment and/or prevention diseases, including diseases affecting the central nervous system.
II. Description of Related Art
The mammalian brain is parsimonious with respect to utilization and salvaging of reduced sulfur (Stryer, 1995). Accordingly, efficient metabolic pathways exist to recycle sulfurous side products of biochemical alkylation reactions (Stryer, 1995; Cooper, 2004). Principal amongst these are the transulfuration pathway (Scheme 1) and the methionine salvage pathway (Stryer 1995; Cooper, 2004; Giulidori et al., 1984). Scheme 1 shows the classical folic acid cycle and transulfuration pathway. The transulfuration pathway is comprised of homocysteine, cystathionine, and downstream products. MS=methionine synthetase; SHM=serine hydroxymethyltransferase; MTR=methylene-THF reductase; CβS=cystathionine β-synthase; CγL=cystathionine γ-lyase; GSH=reduced glutathione; DHF=dihydrofolate; THF=tetrahydrofolate; PC=phosphatidylcholine.
The transulfuration pathway converts homocysteine (hCys) waste from the folate cycle into the intermediate cystathionine, ultimately regenerating cysteine via the tandem actions of cystathionine β-synthase (CβS) and cystathionine γ-lyase (CγL) (Scheme 1). The methionine salvage pathway (not illustrated in Scheme 1) breaks down S-adenosylthiopropylamine derived from S-adenosyl methionine (SAM) flux into the polyamine pathway (Cooper, 2004; Giulidori et al., 1984).
Despite the metabolic effort expended to prevent sulfur leakage, mammalian brain contains relatively high concentrations of non-canonical sulfurous amino acids such as lanthionine and its derivative lanthionine ketimine (LK) (Cavallini et al., 1983; Cavallini et al., 1985; Fontana et al., 1997; Cavallini et al., 1991; Fontana et al., 1990). Lanthionine, LK, TMDC and analogous compounds derived from cystathionine were discovered and measured in mammalian brain during the 1980s and 1990s by Italian researchers lead by Doriano Cavallini and colleagues (Fontana et al., 1997; Cavallini et al., 1985; Cavallini et al., 1983; Cavallini et al., 1991; Fontana et al., 1990; Cooper, 2004). Cavallini demonstrated that LK in particular could bind synaptosomal membranes with 50 nM affinity (Fontana et al., 1997); however he was unable to demonstrate discrete bioactivities inherent to this unusual sulfurous metabolite.
LK is a cyclic sulfurous thioether; the structure of (R)-LK is shown here:
This compound serves no known physiological purpose and are generally considered a waste products of the transsulfuration or methionine salvage pathways. For instance, its precursor, lanthionine, is thought to form during the “misreaction” of CβS (Cooper, 2004). CβS normally condenses serine with hCys (Schemes 1 and 2) but can conjugate serine with cysteine instead. In this case, the product is lanthionine rather than cystathionine (Giulidori et al., 1984; Cavallini et al., 1983). Scheme 2A shows the classical first step of the transulfuration pathway, catalyzed by cystathionine β-synthase (CβS). Scheme 2B shows the alternative reaction which CβS also catalyzes, namely the conjugation of serine (Ser) with cysteine (Cys) yielding lanthionine (αKG refers to alpha ketoglutarate; CγL refers to cystathionine γ-lyase).
In contrast, lanthionine does not act as a substrate for CγL but instead undergoes enzyme-catalyzed reaction with pyruvate to form LK, a process mediated by kynurenine amino transferase (KAT; also known as glutamine transaminase K (GTK) or cysteine conjugate β-lyase (CCβL)), as shown in Scheme 3 (Giulidori et al., 1984; Cavallini et al., 1983).
The resulting intermediate rapidly cyclizes to form lanthionine ketimine (Scheme 3). which can be reduced to the corresponding secondary amine, thiomorpholine dicarboxylic acid (TMDC) (Giulidori et al., 1984; Cavallini et al., 1983).
Prior to this invention, Lanthionine ketimine (LK) is a compound for which little or no therapeutic activity had been defined. Determination of biological efficacy of different lanthionine ketimines was not known in the art because no biological function for endogenous LK had been identified. Hence, prior investigators were unable to design a practical quantitative bioassay against which to test the relative efficacy of various LK-based drugs and pro-drugs.